With the increased attraction of convenience (already at least partially prepared and packaged) foods, especially the fresh or non-frozen foods, that appeal to a modern age populus, the hazards of illness from spoiled food has increased. As distinguished from frozen or cooked products, such foods require a substantially greater degree of control. Aggravating the problems of controlling the quality of such food is the fact that such food may actually be spoiled and such spoilage can go undetected because no foul odor is detectable. Many of these new convenience foods that are being introduced into the market are chilled products that are packaged in a modified atmosphere and contain no preservatives. This creates a potential problem for the manufacturers, distributors, and consumers of these products--a product that is temperature abused can look, smell and taste wholesome, but can be toxic. The only way that such products can be kept safe is to maintain strict temperature control of the product during its entire shelf life. Since it is well known that temperature abuses occur during distribution, an indicia that gives some measure of the intrinsic wholesome quality of a product that is read by the human eye will be of extreme value in guaranteeing the wholesomeness of the product.
While many attempts have been made and various patents have been issued dealing with devices designed to be attached to a package and to show when a package has been temperature abused or has reached the end of its useful shelf life, none is known that is sufficiently adaptable to closely monitor the deterioration of a particular product. For example, an active indicator element can be an acetylenic compound that changes color, an enzyme/substrate system with a pH indicator that changes color, or a dyed wax that diffuses along a strip of paper, to name several. The indicator can be interpreted by visual comparison of the indicator section to a stable reference color(s), or messages can be made to appear from the background. Some indicators rely on electro optical scanning of the indicator, measurement of the indicator resistance, or other machine readable output. Some such methods show how to make a device that changes color when exposed to a cumulative time-temperature exposure. In general, the higher the temperature, the faster the rate of color change and the faster the indicator would signal the end of the life of the product. U.S. Pat. No. 3,999,946, for example, teaches the use of the solid-state polymerization of acetylenes as full time-temperature history indicators. Full time-temperature history indicator composites that are particularly useful for monitoring the shelf lives of perishable products are disclosed in U.S. Pat. No. 4,788,151. The indicator compositions describe therein are shown to be useful in monitoring the shelf lives of products from days to years at room temperature, and from days to years at refrigerated temperatures. In addition, it is shown that these compositions can be formulated into conventional ink systems and printed.
Other methods of preparing full time-temperature history indicators include: U.S. Pat. No. 3,768,876 which incorporates a redox dye in its reduced state (red), that oxidizes as a function of time and temperature in an atmospheric environment and becomes colorless; U.S. Pat. No. 4,292,916 which describes carrier mixtures and receptive layers that react during a given time interval; U.S. Pat. No. 4,212,153 which describes a laminated indicator with at least two layers that gives rise to a perceptible color due to the molecular migration of an agent from an inner layer so an external layer; U.S. Pat. No. 3,344,670 which describes the use of silver nitrate on a bleached paper together with a color comparator to signal integral time-temperature exposures that follow Arrhenius kinetics; and U.S. Pat. No. 3,966,414 which describes the combination of free radical sensitive dyes and peroxides to prepare time-temperature integrators with varying activation energies. The basic similarity and mechanism of action of all previous technologies is for a section of the indicator to change in some single measurable property, due to physical and/or chemical change(s). Indicators are basically grouped into two families, those which signal a change only after a certain critical temperature has been reached or exceeded and those which integrate over the entire temperature range and signal a condition at any stage. The simplest embodiment of the secondary indicator, one which begins to signal above a predetermined temperature, is to have a heat meltable material within which is described a colored dye. When this layer is printed behind the primary indicator system and kept below its melt point, then no migration of the heat fusible layer takes place and hence, no color is developed. Above the melting point of the material, the dyed layer will be mobile and will cause color to be developed in the observed layer. It may be advantageous to have the heat fusible layer and the secondary indicator layer be individual components of a co-reactant pair that gives rise to color. This could be through the use of pH sensitive materials, oxidizable or reducible dyes, organic molecules with functional groups that react to form or change color, metal ions and chelating agents, or the like. A preferred embodiment of the secondary indicator system is one in which a water soluble dye is dispersed in a solvent based ink. This dye system is essentially colorless, or only lightly colored, until it is contacted with a hydrophilic solvent for the dye composition. Once contacted with the hydrophilic solvent, the dye dissolves and produces a bright color in the visible layer. Convenient dyes include FD&C approved dyes. A preferred heat fusible material that solubilizes these dyes is polyethylene glycol (PEG). PEG comes in a variety of molecular weights. The higher the molecular weight the higher the melt point of the system. Virtually any melt point from freezer temperature through 52.degree. C. can be obtained by mixing different proportions of different PEG's. Methods of preparing indictors that react above a predetermined temperature are described in U.S. Pat. Nos. 4,432,656; 4,643,588; 3,065,083; 4,057,029; and 3,967,579. Although descriptions of many different concepts and methods of preparing indicators have been published, only a limited number of such systems have received any commercial interest.
Of the available technologies that have attained some practical utility, none address the problem in a sufficient manner. In general, most chemical reactions leading to the loss of quality of a product follow an Arrhenius-type temperature dependence. This is true of most of the time-temperature sensitive devices described in the literature. As shown in the drawing and described hereafter, an Arrhenius-type plot of the time for a certain indicator to read a particular color as a function of time and temperature has been illustrated. However, it is known that in many instances that the growth rate of microorganisms does not follow a strictly Arrhenius temperature dependence, and in fact some microorganisms do not grow below a certain temperature in the chill range. Organoleptic and/or textural properties of the product would limit the useful life of a product when stored under proper refrigerated conditions. Also shown by comparison is an illustration of a specific product, i.e. raw fish by way of example, and the time to toxin production for such raw fish stored in a modified atmosphere. The data show that the temperature dependence of the reaction for this particular product is clearly non-Arrhenius (non-linear on the plot). Thus, a device that follows an Arrhenius relationship would not be adequate to monitor the storage conditions of a product when the quality of whose temperature dependence is clearly non-Arrhenius (non-linear on the plot). The superposition of the curves of the two data sets illustrate areas of disparity of the actual condition relative to the theoretical (Arrhenius curve) and show that products stored at the proper temperature would be prematurely signaled as having reached the end of the shelf life well before the actual end of the life of the product, thus causing a perfectly good product to be unnecessarily discarded. Even if the slope of the line is changed by varying the mode of action of the device, there will be an unsatisfactory compromise between the indicator response and the end of the useful or safe life of the product.
Another known indicator is that described as a dyed wax device. This indicator is comprised of a dyed wax that melts at a certain temperature and migrates down a wick to signal the amount of time above a threshold temperature. An indicator such as this could be used to signal high temperature abuse, but would give no practical signal during long term proper cold storage. Thus, a properly stored product using this type of indicator would never be shown to be spoiled, even though all chilled products have a finite lifetime during cold storage. In general this type of diffusion process is not very temperature sensitive and has a shallower slope on the Arrhenius plot.